RE: pen reverse

["Alain Tougas" <alain@xxxxxxx>]

Hi,

Unfortunately, px is absent in most versions of Logo (including MicroWorlds
EX).

px was a special pen state (like pu pd and pe) that would draw where there
was no line, and erase where there was one. It could produce somewhat
predictable results in a monochrome environment, but not in a color world:
what do you get when you px on red. and then on pink. Black and white was a
simple case of "turn on or turn of this pixel".

What you are looking at in this example is a moire effect due to the fact
that the lines are close to one another towards the center, and less so
towards the perimeter. So px would do a reversal of the video near the
center, and not towards the perimeter.

The fact that fd 40 fd 40 does not produce the same result as fd 80 seems to
indicate that the moire effect is augmented by the lack of precision of this
particular version of logo. In theory (and in all MicroWorlds versions), fd
40 fd 40 does the same as fd 80.

In other words, if you try this in different versions of logo, you would get
different results, perhaps even uninteresting results.

In some very old versions of Logo (Apple Logo), the computer's architecture
itself had some flakiness that produced this kind of art. The computer was
only capable of producing 4 colors, and an additional 4 colors were produced
by turning on "the neighbouring pixel". So a red line might have been made
of two lines, none of them being red. Px was very unpredictable then (but
very artzy).

Ciao gang,

Alain



-----Original Message-----
From: mwforum-admin@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
[mailto:mwforum-admin@xxxxxxxxxxxxxxxxxxxxxxxxxxxx]On Behalf Of Burke,
Bridget -CKJH
Sent: February 23, 2007 12:33 PM
To: mwforum@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Subject: pen reverse

A student saw this idea about pen reverse on this website. Can anyone
explain this idea?

http://www.mathcats.com/gallery/15wordcontest.html
<BLOCKED::http://www.mathcats.com/gallery/15wordcontest.html>